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ORNL Restores US Capability To Produce Plutonium-238 (ornl.gov)
hypnosec writes: Oak Ridge National Laboratory has successfully produced 50 grams of plutonium-238, an isotope that produces heat without a lot of other, problematic radiation. This makes it suitable for use in radioisotope thermoelectric generators, which can power space probes. The new sample effectively revives the U.S.'s end-to-end plutonium-238 production capabilities, which have been dormant for around 30 years since work was stopped at the Savannah River Plant in South Carolina. The ORNL is optimistic this important milestone will pave the way for regular production of the material, ensuring constant supply for NASA's future missions.
Most if not all of the Soviet-era lighthouse RTGs used Sr-90, an isotope of strontium rather than Pu-238 as a heat source. It required heavier shielding than a Pu-238 RTG but in land-based generators the extra mass of the case didn't affect its capabilities the way an RTG to be mounted on a spacecraft would.
Sr-90 can be sourced from spent fuel from power plants and the Soviets had a fuel reprocessing capability to produce Sr-90 in quantity. The Russian government is looking to upgrade and expand their existing fuel reprocessing operations, in part to supply their next-generation series of fast reactors like the BN-800 with recycled spent fuel.